Production of short pulses with high energy per pulse is usually achieved by a combination of one oscillator and one amplifier. The oscillator is traditionally a mode-locked laser producing very short pulses, typically less than 100 ps, at high frequency, typically a few tens of MHz, and with low energy per pulse, typically a few nJ. To increase the pulse energy to several μJ, an amplifier working at a lower repetition rate, ranging from a few kHz to a few hundreds of kHz depending on the pumping configuration, is used. Unfortunately, the traditional systems are complex and complicated to use because they involve active modulation (acousto-optic or electro-optic), high-speed electronics, short-pulse production for the oscillator, and injection and synchronization of the pulses inside the amplifier.
Passively Q-switched lasers using Nd-doped crystals can produce high peak power pulses of several kW at a wavelength of 1064 nm. Depending on the experimental setup, the pulse width can vary from a few tens of ns (A. Agnesi, S. Dell'Acqua, E. Piccinini, G. Reali and G. Piccinno, “Efficient wavelength conversion with high power passively Q-switched diode-pumped neodymium laser”, IEEE, J. Q. E., Vol. 34, 1480–1484, 1998) to a few hundreds of ps (J. J. Zayhowski, “Diode-pumped passively Q-switched picosecond microchip lasers”, Opt. Lett., Vol. 19, 1427–1429, 1994). For example, pulses of 19 ns and 108 μJ can be obtained at 25 kHz and 1064 nm from a diode-pumped Nd:YAG laser with a Cr4+:YAG saturable absorber crystal. The high peak power of these lasers allows efficient wavelength conversion into the ultra-violet (UV) range with optically nonlinear materials (A. Agnesi, S. Dell'Acqua, E. Piccinini, G. Reali and G. Piccinno, “Efficient wavelength conversion with high power passively Q-switched diode-pumped neodymium laser”, IEEE, J. Q. E., Vol. 34, 1480–1484, 1998; J. J. Zayhowski, “Diode-pumped passively Q-switched picosecond microchip lasers”, Opt. Lett., Vol. 19, 1427–1429, 1994; J. J. Zaykowski, “UV generation with passively Q-switched microchip laser”, Opt. Lett., Vol. 21, 588–590, 1996).
To reduce the pulse width, while using the same material combination, one must combine the active medium and the saturable absorber in a short distance to reduce the cavity length to about 1 mm. A microchip laser combines the two materials in a monolithic crystal (J. J. Zaykowski, “Non linear frequency conversion with passively Q-switched microchip lasers”, CLEO 96, paper CWA6, 23 6–237, 1996) to reduce the energy to approximately 8 μJ at 1064 nm. The two materials, i.e. the laser material and the saturable absorber, can be connected by thermal bonding, or the saturable absorber can be grown by liquid phase epitaxy (LPE) directly on the laser material (B. Ferrand, B. Chambaz, M. Couchaud, “Liquid Phase Epitaxy: a versatile technique for the development of miniature optical components in single crystal dielectric media”, Optical Materials 11, 101, 1998). At the same time, in order to obtain sub-nanosecond pulses, the saturable absorber must be highly doped to lower the repetition rate, e.g. 6–8 kHz with Nd:YAG. The wavelength conversion efficiency from infrared (IR) to UV is in the order of 4%. A solution to simultaneously obtain short pulses and a high repetition rate is to combine a Nd:YVO4 crystal, whose short fluorescence lifetime is well suited for a higher repetition rate, with a semiconductor-based saturable absorber in an anti-resonant Fabry-Perot structure (B. Braun, F. X. Kdarner, G. Zhang, M. Moser, U. Keller, “56 PS passively Q-switched diode-pumped microchip laser”, Opt. Lett. 22, 381–383, 1997). Unfortunately this structure is nevertheless complex and very difficult to produce.
It is therefore difficult to simultaneously produce sub-nanosecond short pulses, at frequencies of a few tens of kHz, with several micro-Joule per pulse in a simple and compact system. Another solution consists of combining a compact oscillator, producing short pulses at high frequency, with an amplifier to increase the pulse energy. Amplifiers have been used in the past with pulsed microlasers. After amplification, pulses with 87 nJ (small-signal gain of 3.5) at 100 kHz have been produced using a 10-W diode bar as a pump (C. Larat, M. Schwarz, J. P. Pocholle, G. Feugnet, M. Papuchon, “High repetition rate solid-state laser for space communication”, SPIE, Vol. 2381, 256–263). A small-signal gain of 16 has been obtained with an 88-pass complex structure using two 20-W diode bars as a pump (J. J. Degnan, “Optimal design of passively Q-switched microlaser transmitters for satellite laser ranging”, Tenth International Workshop on Laser Ranging Instrumentation, Shanghai, China, Nov. 11–15, 1996). In these two examples, the amplification efficiency, which can be defined as the ratio between the small-signal gain and the pump power, is small because the transverse pumping has a low efficiency due to the poor overlap of the gain areas with the injected beam. Furthermore, these setups use Nd:YAG crystals not suited for high-frequency pulses (the fluorescence lifetime is 230 μs).
A combination of Nd ions in two different hosts, in an oscillator-amplifier system, has been performed in the past in continuous wave (CW) (H. Plaesmann, S. A. Re, J. J. Alonis, D. L. Vecht, W. M. Grossmann, “Multipass diode-pumped solid-state optical amplifier”, Opt. Lett. 18, 1420–1422, 1993) or pulsed mode (C. Larat, M. Schwarz, J. P. Pocholle; G. Feugnet, M. Papuchon, “High repetition rate solid-state laser for space communication”, SPIE, Vol. 2381, 256–263). In these cases, the spectral distance between the emission lines of the two different materials, i.e. Nd:YAG and Nd:YVO4, limits the small-signal gain to a value lower than that obtained when only Nd:YVO4 is used in both the oscillator and the amplifier; the aforementioned spectral distance is comprised between 5.5 cm−1 and 7.0 cm−1 (J. F. Bernard, E. Mc Cullough, A. J. Alcock, “High gain, diode-pumped Nd:YVO4 slab amplifier”, Opt. Commun. Vol. 109, 109–114, 1994).
A number of amplification schemes using Nd ions in crystals have been studied, but often end up with complex multipass setups, with low efficiency due to transverse pumping.
End-pumped single-pass or double-pass amplification schemes based on guiding structures to increase the interaction length between the pump beam and the injected beam have been studied in the past: in planar guides (D. P. Shepherd, C. T. A. Brown, T. J. Warburton, D. C. Hanna and A. C. Tropper, “A diode-pumped, high gain, planar waveguide Nd:Y3Al5O12 amplifier”, Appl. Phys. Left., 71, 876–878, 1997) or in double-cladding fibers (E. Rochat, K. Haroud, R. Dandliker, “High power Nd-doped fiber amplifier for coherent intersatellite links”, IEEE, JQE, 35, 1419–1423, 1999; I. Zawischa, K. Plaman, C. Fallnich, H. Welling, H. Zellner, A. Tunnermann, “All solid-state neodymium band single frequency master oscillator fiber power amplifier system emitting 5.5 W of radiation at 1064 nm”, Opt. Lett. 24, p. 469–471, 1999). However, these schemes are not suited for high-peak-power pulses because unwanted nonlinear effects, such as the Raman effect, start to appear around 1 kW of peak power.
A high small-signal gain of 240 was achieved in an end-pumped double-pass bulk Nd:YLF amplifier, but it was used with a CW laser with an expensive diode-beam shaping optical setup (G. J. Friel, W. A. Clarkson, D. C. Hanna, “High gain Nd:YLF amplifier end-pumped by a beam shaped bread-stripe diode laser”, CLEO 96, paper CTUL 28, p. 144, 1996).
U.S. Pat. No. 6,373,864, Georges et al., issued Apr. 16, 2002, incorporated herein by reference, discloses an entirely passive laser system both for the generation and amplification of short pulses. In the Georges et al. invention, the oscillator directly produces μJ pulses at the required repetition rate, and the pulses are amplified after only a few passes in a non-synchronized amplifier. The uniqueness of that approach was to combine an optically pumped, passively Q-switched, high frequency, Nd:YAG microchip laser producing short pulses with an optically end-pumped Nd:YVO4 amplifier producing high small-signal gain while pumped at low power. The use of the two materials, Nd:YAG and Nd:YVO4, allowed the best use of their respective properties: Nd:YAG/Cr4+:YAG microchip lasers are simpler and easier to manufacture than Nd:YVO4 microchips because they use the same crystal (YAG) for the laser medium and the saturable absorber, and can be produced in a collective fashion. In addition they produce shorter pulses except in the case of the semiconductor saturable absorber described in B. Braun, F. X. Kartner, G. Zhang, M. Moser, U. Keller, “56 ps passively Q-switched diode-pumped microchip laser”, Opt. Lett. 22, 381–383, 1997. Nd:YVO4 is on the other hand well suited for amplification due to its high stimulated emission cross section. It is also better suited than Nd:YAG for higher repetition rates due to a shorter fluorescence lifetime (100 μs instead of 230 μs).
In the invention disclosed be Georges et al., the light beam to be amplified initially gets passed through the amplifier medium along a first path and subsequently gets reflected back through the amplifier medium along a second path, thereby traversing the amplifier medium twice. The planar geometry used by Georges et at is not optimal since the pump beam propagates in three dimensions whereas the light beam to be amplified travels in a single plane. This results in poor overlap between the volume occupied in the amplifier medium by the pump beam and the volume occupied in the amplifier medium by the light beam to be amplified. Georges et al. alludes to multi-pass scenarios wherein the light beam (to be amplified) traverses the amplifier medium at least twice. Such multi-pass amplification schemes are known. For instance, McIntyre discloses co-linear and two-dimensional multi-pass amplification schemes in U.S. Pat. No. 5,268,787, issued Dec. 7, 1993, which is incorporated herein by reference. Plaessmann et al., in U.S. Pat. No. 5,546,222, issued Aug. 13, 1996, which is incorporated herein by reference, discloses a multi-pass laser amplifier that uses optical focusing between subsequent passes through a single gain medium. The multi-pass laser amplification schemes disclosed by Plaessman et al. are all two-dimensional schemes, i.e. the multi-paths of the light beam traversing the amplifier medium all lie in a same plane. The number of optical components used in the embodiments taught by Plaessman et al. is relatively small and consequently, the alignment of said components is crucial in view of the multi-pass amplification scheme.
Three-dimensional amplification schemes are also known. C. LeBlanc et al., “Compact and efficient multi-pass Ti:sapphire system for femto-second chirped-pulse amplification at the terawatt level”, Optics Letters, Vol. 18, No. 2, Pp. 140–142, Jan. 15, 1993, discloses a Ti:sapphire crystal amplifier medium pumped at two ends by Nd:YAG light and traversed eight times by the light beam to be amplified. The light beam to be amplified traverses the amplifier medium four times in a first plane and four other times in a distinct second plane parallel to the first plane. Another three-dimensional amplification scheme is that of Scott et al., “Efficient high-gain laser amplification from a low-gain amplifier by use of self-imaging multi-pass geometry”, Applied Optics, Vol. 40, No. 15, Pp. 2461–2467, 20 May 2001. Scott et al. illustrates how the light beam to be amplified traverse the amplifier medium four times in a first plane and four additional times in a distinct other plane parallel to the first plane. A phase-conjugate mirror is then used to double the number of passes.
The three-dimensional amplification schemes discussed above are quite complex and not well suited for miniaturization.